The present invention pertains generally to communication data links. More particularly, the present invention pertains to transceivers which are useful in terrestrial free-space optical data links. The present invention is particularly, but not exclusively, useful as a method, or a system, for optically aligning a transceiver at a first location with a transceiver at a second location.
A basic requirement in all modern day societies, in both the government and private sectors, is the ability to effectively and accurately communicate between two or more locations. As is well known, such communication can be accomplished in any of several different ways. For example, wire communications such as a telephone exchange, may be either automatic or manual. The common characteristic of wire communication systems, however, is that they have a continuous connection through a system. On the other hand, wireless communications systems, such as radio, have no connecting wires and, instead, involve methods of signaling through space by means of electromagnetic waves. Not surprisingly, communication systems which incorporate interconnecting links of wire and wireless components have also been used.
In recent years, optics have been used in an ever-increasing number of applications for both wire and wireless type links in communication systems. One particularly popular component which is used as a wire link in an optical communications system is the optical fiber. In general, an optical fiber is made of ultra-pure glass and has a central core having a higher refractive-index glass than its outer cladding. Importantly, with this construction an optical fiber is capable of conducting modulated light signals by total internal reflection. Depending on the particular application, an optical fiber may be either a single mode fiber or a multi-mode fiber. More specifically, for a single mode fiber, the diameter of the inner core is comparable with the wavelength of the transmitted light. Thus, there is only one mode of light propagation through the fiber, and distortion is minimized or eliminated. In comparison, a multi-mode optical fiber has a core diameter which is sufficiently larger than the wavelength of the transmitted light. This allows propagation of the light energy in a large number of different modes. Typically, multi-mode optical fibers are used as wire links for data networks which are located inside of buildings where high loss, and therefore short distances, can be tolerated. Further, multi-mode optical fibers are relatively inexpensive and are easier to couple into and out of with other network components than are single mode fibers. High speed telecommunications, however, often require the use of single mode optical fibers which have less loss and which are useable over longer distances than are the multi-mode optical fibers. The single mode fibers, however, are relatively more expensive and are harder to couple into and out of with other network components. Thus, the trade-offs can be significant.
Regardless of whether a single mode or a multi-mode optical fiber is used, in all instances the benefits of using an optical fiber include small diameters, high potential bandwidth and lower costs. Further, optical fibers can be used either alone, or they can be bundled into cables. In certain respects, however, an optical fiber communications system suffers the same shortcomings as any other wire system. Namely; not all communications systems are able to effectively and efficiently incorporate optical fibers throughout the entire system. Indeed, optical communications links across free-space may be necessary and desired.
The question of incorporating a terrestrial free-space optical data link for use with an optical fiber system raises several interesting issues. These include: the possible need for amplification of the optical signal before its transmission from the optical fiber across the free-space; the need to maintain the integrity of the optical signal during its transmission across the free-space; and, the detection, alignment, and focusing of the optical signal after it has been transmitted across the free-space. As implied above, the use of optical fibers in a communication system means that the communication beam will have an extremely small diameter. Multi-mode optical fibers, for instance, can have core diameters as small as about fifty microns. Single mode fibers are even smaller, and can have core diameters of only around ten microns. The ability to focus light into such a small diameter after propagation through free space in a manner which will allow for effective transmission of the data carried on the beam is of no small concern. The problems are further complicated by the fact that effective wavelengths for communication beams are longer than the wavelength of visible light and, therefore, require expensive, specialized equipment for their detection. Additionally, the use of longer wavelengths for the communication beam is preferable due to the fact that amplification of the light beam is more easily accomplished at such wavelengths.
In light of the above it is an object of the present invention to provide a method and a system which are capable of continuing the high speed telecommunications data transmitting capacity of an optical fiber over a terrestrial free-space optical data link. Another object of the present invention is to provide a system and a method which will interconnect an optical fiber with other optical elements for the uninterrupted transmission of a communication beam between the optical fiber and another medium, such as the earth""s atmosphere. Still another object of the present invention is to provide a system and a method which will align a communication beam with an optical target, such as an optical fiber or an optical detector, wherein the target has a reception area that is less than approximately two hundred microns in diameter. Yet another object of the present invention is to provide a system and a method for directing a communication beam over a terrestrial free-space optical data link which is easy to manufacture, relatively simple to use, and comparatively cost effective.
In accordance with the present invention, a system for directing a communication beam from a first location through a transit distance to a second location over a terrestrial free-space optical data link includes an optical fiber which emanates the communication beam at the first location. Preferably the wavelength of the communication beam is in the intermediate infrared, and is approximately 1550 nm, so that it may be amplified by an erbium doped optical amplifier before being transmitted over the data link.
A transmitter is positioned at the first location and is optically connected with the optical fiber. Specifically, the transmitter includes an optical element for collimating and directing the communication beam along a path toward a target at the second location. Additionally, the transmitter at the first location includes a laser diode for generating a beacon beam which is also optically connected with the optical element of the transmitter. The optical element of the transmitter thus collimates and directs both the beacon beam and the communication beam along the path as a combined common beam. In one embodiment of the present invention a coupler is used to couple the beacon beam directly with the communication beam in the optical fiber before the two beams emanate from the optical fiber. In another embodiment, the transmitter transmits the communication beam along a first path while it transmits the beacon beam along a second path. For this embodiment, the second path is substantially parallel to the first path so that there is an overlap region which is established beyond an overlap distance from the transmitter. Once the beams overlap beyond the overlap distance, a portion of the communication beam and a portion of the beacon beam will then travel further on the path as the common beam. Importantly, for either embodiment, both the communication beam of one wavelength and the beacon beam of another wavelength are combined into a common beam before the common beam arrives at the second location.
At the second location there is a receiver which is positioned to collect the common beam and to then focus the common beam to a focal point on an optical target. Included in the receiver at the second location is a camera which receives scattered light from the optical target which will include an image of the focal point. By detecting this scattered light, the camera is able to locate the focal point. For the present invention the camera can be a CCD type camera, a CMOS type camera, or any other similar type camera which will detect visible or near infrared light from the beacon beam. Preferably, the camera is relatively inexpensive.
As used in the present invention, based on the known location of an optical target on the detector in the receiver, and the detected location of the focal point of the received common beam, the camera determines any displacement distance which may exist between the focal point of the common beam and the optical target. As contemplated for the present invention, the optical target can be the core of a single mode optical fiber wherein the core has a diameter of approximately ten microns (10 xcexcm). Alternatively, the target can be part of a larger optical detector or multi-mode fiber having a diameter in a range of approximately thirty to seventy microns (30-70 xcexcm). In either case, the objective of the present invention is to then reduce the displacement distance between the focal point of the common beam and the optical target to a null. In effect this will make the focal point of the common beam incident on the optical target.
In order to align the focal point of the common beam with the optical target, an adjuster is used which will move the focal point relative to the target. For one embodiment of the present invention, the adjuster can be a mechanical linkage which is manually operable to position the focal point of the common beam on the target. For another embodiment of the present invention, the alignment of the focal point with the optical target can be accomplished using a computer. If used, the computer will be programmed to receive input from the camera and to generate an error signal that is indicative of the displacement distance between the focal point of the common beam and the target. Further, the computer will also include electronics for activating the adjuster to reduce the error signal to a null. The computer can be used thereafter to substantially maintain the error signal at the null.
It is contemplated that the data communication link which is established by the present invention may be either a one-way communication link, such as described above, or a two-way communication link. For the latter case, the system can include a first base on which are mounted: a transmitter, a beacon beam generating laser diode, a receiver, a camera and an adjuster with or without an associated computer. This first base can then be positioned at the first location and optically connected with an optical fiber which will provide the out-going communication beam and an optical fiber which will receive the in-coming communication beam. At the second location there will then be a second base on which are mounted a transmitter, a beacon beam generating laser diode, a receiver, a camera and an adjuster with or without an associated computer. Similar to the first base, the second base can be connected with optical fibers which will provide the out-going communication beam and receive the in-coming communication beam. The two-way communication link can thus be established between the first location and the second location.